A large part of the traffic load in future wireless communication systems is expected to originate from indoor users, for example from users in office buildings, cafés, shopping malls etc. Providing the indoor users with high bit-rate and spectrally efficient communication from outdoor base stations is challenging due to the penetration loss that is experienced by signals propagating through building walls. One well known solution for enhancing the indoor coverage is to use outdoor-to-indoor repeaters. An outdoor-to-indoor repeater has a pick-up antenna on the outside of the building connected via a double-directional power amplifier to a donor antenna on the inside of the building. Another well known solution is to deploy pure indoor systems for example by deploying an indoor radio base station (RBS) and connect it to a distributed antenna system (DAS) where the antennas are also located indoors and close to the users. An alternative to using DAS is to use leaky (coaxial) cables. Typical use cases for leaky cables are indoor deployments and along railway tunnels. Put simply, a leaky cable is a coaxial cable with slots or gaps along its entire length which enable the cable to “leak” electromagnetic waves. The leaky cable exhibits radiation properties different to a DAS, such as for example having more uniform signal levels over the coverage area. A leaky cable can be used both for transmitting and receiving electromagnetic waves, i.e. it allows two-way communication.
Leaky cables are relatively expensive and complicated to install due to their weight and stiff profile. In multi-stream applications such as multiple-input multiple-output (MIMO) communications multiple cables, in particular one leaky cable for each stream, need to be installed more or less in parallel which complicates the installation even more. MIMO communications are typically applicable indoors where high bit rates are sought after, as indoor areas are often rich scattering environments suitable for MIMO. A standard leaky cable leaks energy along its entire length and it has a large attenuation per meter which means that the Signal to Noise ratio (SNR) experienced by a user device located close to the end of the leaky cable is much less than for a user device located at the beginning of the leaky cable, at which end the leaky cable is fed. This leads to a very skew capacity distribution along the leaky cable which is much undesired. One way to combat this skewness is to introduce multiple Radio Frequency (RF) amplifiers or repeaters along the leaky cable that can amplify the signal propagating through the leaky cable. A drawback is that such a solution is expensive and that power sources at each installation point are needed, which again complicates the installation and increases the cost. Another way to combat the skewness is to equalize the power radiated per length unit by increasing the number of perforations towards the end of the leaky cable.
Leaky cables are often used to cover long areas such as tunnels, corridors etc. For deployment of higher order MIMO, several leaky cables have to be installed in parallel which is costly. For instance, for 8×8 MIMO, eight leaky cables have to be installed through the entire length of the area. As mentioned, these leaky cables are very costly as well as hard to install and the above described difficulty of maintaining the same capacity over the entire length of the leaky cable is also encountered.
Patent publication US 2013/0162500 describes the connecting of two antenna ports of a network node to a single leaky cable, thereby achieving improvements for 2×2 MIMO deployment. For instance, the teachings therein provide a more evenly distributed capacity along the cable and also enable reduction of the number of required cables for a given MIMO order. Although providing a well-functioning solution advantageously applied for circular coverage areas, such solution is not equally well suited for parallel leaky cable deployments, e.g. in straight long corridors or long tunnels for the above mentioned reasons. For example, implementing a solution according to the mentioned publication in a corridor would give no improvement e.g. in view of cable length; although four cables could be used for the 8×8 MIMO example, the total length would still be the length of 8 cables following the corridor to one end and then back again.